Technical Field
The invention relates to the treating of wastewater to produce a chemically acceptable effluent stream. More particularly, the invention relates to an automated system and method for treating wastewater that controls contaminant levels of a plurality of contaminants present in the wastewater.
Description of the Background Art
The treatment of industrial wastewater is a necessary and difficult task common to most, if not all manufacturing facilities. A vast array of contaminants that are byproducts of manufacturing processes may be removed from the process by a wastewater stream. Accordingly, this wastewater stream carries contaminants in the form of suspended solids that range widely in size, as well as an assortment of liquids, oils, surfactants, polymers, acids, fats, blood, process ingredients, metal salts, total suspended solids (TSS), biological oxygen demand (BOD), chemical oxygen demand (COD), and the like. It is therefore necessary, and typically required, to remove and/or neutralize these contaminants to maintain an effluent stream that meets the minimum standards of wastewater sewage for a given locale prior to releasing the effluent into a sewage system.
A variety of prior art systems for wastewater treatment employing various technical approaches have attempted to solve these problems. Systems employing tanks and weirs for separation of contaminants from liquids through sedimentation have been in widespread use. Sedimentation in holding tanks is often accompanied by utilizing filtration systems, such as filters or screens, to remove small solid particles in the effluent stream. However, filtration systems require a great deal of maintenance and are subject to clogging or partial clogging, thereby impeding flow through a system.
Dissolved air flotation systems (DAF's) have been employed with some degree of success. In DAFs, air bubbles are introduced at a lower portion of a flotation tank to carry particles suspended in the liquid to the surface thereof. The solids at the surface aggregate together, either naturally or through the use of coagulant additives, thereby permitting removal of at least a portion of the solids in the wastewater. Effluent is then drawn from a lower portion of the DAF system. Furthermore, a variety of flotation systems are used where the solids being removed have densities close to that of water.
Additionally, in many wastewater systems the pH of the wastewater stream must be modified to an acceptable level by the addition of acid or bases into the wastewater stream, and frequent testing of pH levels of the effluent to maintain proper pH balance.
Chemical additives are typically required to treat a contaminated wastewater stream. Plant operators typically conduct jar tests wherein a plurality of jars or containers are filled from the wastewater stream, and each is treated with a differing chemical additive, or alternatively a combination of chemical additives, in differing amounts. When multiple additives, such as coagulants and polymers are required to treat the wastewater, a plurality of jar tests are required to test various combinations of additive amounts to determine which combination results in an effluent that is acceptable for discharge from the system.
The requisite amount of chemical additives necessary to treat the wastewater are then recorded, and the flow rate of each additive must then be calculated based upon the rate of flow of wastewater into the treatment system. Once the proper flow rates are established the chemical additives are typically supplied to the system by pumps, which must be set to deliver the proper additive flows.
One great difficulty of this system of wastewater treatment is that, if the wastewater input stream changes appreciably, the additives must be readjusted, thereby requiring additional jar tests. Additionally, a change in wastewater stream flow requires the pumps to be adjusted because the proportion of additives must be adjusted proportionally to the wastewater stream flow. Furthermore, in many manufacturing environments it is impractical to train personnel to monitor the effluent stream, conduct periodic and frequent jar tests, calculate additive flow rates, make pump adjustments, and carefully monitor incoming stream flow rates in order to maintain the treatment system's operation.
Attempts have been made to automate this process (see, for example, U.S. patent publication no. 2012/0000859), but the results obtained using such systems have been less than satisfactory. Such systems use a programmable logic controller (PLC) to perform jar tests that derive a water treatment chemical dosage. This dosage is then implemented and maintained via a proportional feed, i.e. chemical-to-water flow. A nephelometer is used in such system to measure turbidity in Nephelometric Turbidity Units (NTU). The NTU meter and controller then function to provide an output value that is used to start another jar test, and the process repeats. Such system does not provide real time chemical additions. Thus, while such system, especially with the up-front NTU meter, may cut down the back feed time for providing clarity values to the controller, it uses that time and more due to the need to wait for the completion of the jar tests. Such delay compromises both the accuracy of any measurements made and the integrity of removal of contaminants from the effluent stream.
Accordingly, there is a need for an automated wastewater treatment system that is capable of monitoring an effluent stream for out of range contaminant levels and for adjusting additive flow rates accordingly.